Program.cs

using System;
using System.Net;
using System.Net.WebSockets;
using System.Text;
using System.Threading;
using System.Threading.Tasks;

class Program
{
    static async Task Main(string[] args)
    {
        HttpListener httpListener = new HttpListener();
        httpListener.Prefixes.Add("http://localhost:5000/");
        httpListener.Start();
        Console.WriteLine("Listening...");

        while (true)
        {
            HttpListenerContext context = await httpListener.GetContextAsync();
            if (context.Request.IsWebSocketRequest)
            {
                HttpListenerWebSocketContext wsContext = await context.AcceptWebSocketAsync(null);
                WebSocket webSocket = wsContext.WebSocket;

                await Echo(webSocket);
            }
            else
            {
                context.Response.StatusCode = 400;
                context.Response.Close();
            }
        }
    }

    private static async Task Echo(WebSocket webSocket)
    {
        var buffer = new byte[1024 * 4];
        WebSocketReceiveResult result = await webSocket.ReceiveAsync(new ArraySegment<byte>(buffer), CancellationToken.None);

        while (!result.CloseStatus.HasValue)
        {
            var receivedMessage = Encoding.UTF8.GetString(buffer, 0, result.Count);
            Console.WriteLine("Received: " + receivedMessage);

            var serverMessage = Encoding.UTF8.GetBytes("Echo: " + receivedMessage);
            await webSocket.SendAsync(new ArraySegment<byte>(serverMessage), result.MessageType, result.EndOfMessage, CancellationToken.None);

            result = await webSocket.ReceiveAsync(new ArraySegment<byte>(buffer), CancellationToken.None);
        }

        await webSocket.CloseAsync(result.CloseStatus.Value, result.CloseStatusDescription, CancellationToken.None);
    }
}